1. Field of the Invention
The present invention relates to in-vehicle audio apparatuses, and in particular, relates to a method for correcting audio signals in response to the level of noise that occurs in a vehicle that is being driven.
2. Description of the Related Art
In a case where a passenger listens to music through a compact disk (CD), a digital versatile disk (DVD), a radio broadcast, or the like in a vehicle cabin, when the passenger is exposed to background noise, such as an engine noise or a road noise, the passenger cannot hear music in a satisfactory condition. In view of this problem, a system exists in which the noise level is measured with a microphone provided in the vehicle cabin and an optimum correction (for example, correction of the volume) is performed in response to the noise level. Specifically, in this system, a microphone is used to collect sound in a vehicle cabin, and audio signals in external sound signals received by the microphone are cancelled to extract noise.
An apparatus that automatically adjusts the volume in response to noise is disclosed in Japanese Unexamined Patent Application Publication No. 10-335960. This apparatus includes a noise-signal extracting unit that extracts only noise signals from sound signals in a vehicle cabin detected by a microphone and a volume-adjusting unit that adjusts the volume of an audio apparatus in response to the noise signals detected by the noise-signal extracting unit, and this apparatus enables a passenger to hear audio signals from an audio signal source at an appropriate volume in consideration of the noise level in the vehicle cabin.
However, in the known in-vehicle audio apparatus, although played-back music can be distinguished from noise, a speech sound of a person in a vehicle cabin cannot be distinguished. Thus, correction is performed on noise that includes a speech sound. The frequency spectrum and volume of a speech sound of a person can suddenly and markedly change. Thus, when audio signals are corrected on the basis of the level of noise that includes a speech sound, a change in the amount of correction is inevitably large, resulting in an unnatural sound.